kissjxp;
What is the application? There are no specific requirements related to material thickness where you are mandated to switch from single vee to a double vee full penetration weld joint detail. A double vee weld joint can provide reduced distortion during and after welding depending on sequence of welds and can allow back grinding to assure full penetration. A single vee groove can be limited by access to the root resulting in excessive weld deposits, depending on the weld process. Electroslag welds use a square groove weld detail that can extend for 6" in depth with weld deposition performed from one side (similar to a vee groove).
That's mostly an economic decision on your part--increased handling to get to both sides (and even more handling to balance the passes to avoid distortion) vs. larger weld required for single-sided weld (and other means to deal with distortion).
If the product standard or welding code that you are using does not state any requirements, consult the appropriate part of ISO 9692 for the process and/or material under consideration.
